sync: Remove checking of the extrinsics root (#5686)

With the introduction of `system_version` in
https://github.com/paritytech/polkadot-sdk/pull/4257 the extrinsic root
may also use the `V1` layout. At this point in the sync code it would
require some special handling to find out the `system_version`. So, this
pull request is removing it. The extrinsics root is checked when
executing the block later, so that at least no invalid block gets
imported.